Biomasses as referred to mean biological renewable resources, and can be defined as “renewable and biological organic resources, except for fossil resources”. Effective utilization of unutilized plant-derived biomasses among these biomasses such as: timbers, e.g., timbers from forest thinning; rice straws, wheat straws, rice hulls, stalks of starch-producing crops such as maize and sugarcane; empty fruit bunches (EFB) of Elaeis guineensis, etc., has been desired.
Among the components of such plant-derived biomasses, many polysaccharides such as starches are easily degraded by an enzyme or the like to monosaccharides, and utilized as energy sources, as well as foodstuffs, and the like. Accordingly, for the effective utilization of plant-derived biomasses, it is known that degradation of cellulose contained in plant cells at high proportions into methane and/or monosaccharides (glucose) to permit utilization as energy sources as well as foodstuffs and the like would be important. However, as is seen from the fact that cellulose constitutes the vast majority of cell walls, cellulose has a rigid structure and is resistant against degradation; therefore, effective utilization thereof has been hampered under current circumstances.
Specifically, cellulose has multiple structures in cell walls as shown below. The vast majority of cellulose that constitutes cell walls has a quasicrystal structure, which is referred to as “microfibril”, formed by linearly cohesion. The cellulosic components (i.e., microfibrils) having such a quasicrystal structure are bonded with one another via noncellulosic components such as hemicellulose and lignin. These cellulosic components (i.e., microfibrils) and noncellulosic components are arranged to give a large structure which is generally referred to as “fibril”. The fibrils construct cell walls by lamination into a sheet form, in general. In the cellulosic components (i.e., microfibrils) having the quasicrystal structure as described above, polymer chains of cellulose are strongly linked via hydrogen bonds. Due to the hydrogen bonds, plants can have strong cell walls.
A means for degrading cellulose having such a structure into methane may involve a method in which degradative digestion by an anaerobic microorganism is allowed, and the like. However, degradation of cellulose using a microorganism is unsatisfactory in terms of practical applicability on the reasons that controlling the reaction is complicated, and the like.
On the other hand, according to chemical approaches, it is also possible to hydrolyze cellulose into monosaccharides using a catalyst or an enzyme. The monosaccharides obtained by the chemical degradation of cellulose are, for example, converted into ethanol by fermentation, and can be used as energy sources for preexisting internal combustion engines and turbines. However, it is difficult to efficiently allow the cellulosic biomasses derived from plants to be directly hydrolyzed in the chemical approaches, due to the molecular structure of cellulose in cell walls as described above. Such a disadvantage is believed to result from the rigid structure of cellulose that prevents water, enzyme and the like from entry into the quasicrystal structure, thereby leading to significant retardation of the action of a cellulose-degrading enzyme. In other words, since the enzyme is not able to readily enter into the quasicrystal structure formed by strong linkage via hydrogen bonds, it is impossible to directly degrade glycoside bonds. Therefore, the enzyme can merely degrade the quasicrystal structure of cellulose gradually from the surface, and thus it is impossible to attain high efficiencies in direct hydrolysis of cellulosic biomasses by an enzyme.
Accordingly, a method in which a cellulosic biomass is finely disrupted prior to hydrolysis by an enzyme or the like to produce readily hydrolyzable cellulose was proposed. This method fundamentally uses: a chemical action including gradually hydrating cellulose having the quasicrystal structure, and thus weakening hydrogen bonds between polymer chains of adjacent cellulose by way of the hydration; and a physical action of mechanically imparting a force to the cellulosic biomass by beating, kneading or the like to disrupt the cellulose polymer chains. More specifically, with respect to such a method, for example (1) a technique including agitating cellulosic biomass particles in a vessel to prepare a suspension of the particles, and thereafter elevating the temperature of the suspension of the particles and gradually supplying water to allow for hydration while continuously agitating the particles, whereby fine powders are produced (Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2004-526008); (2) a technique including mixing cellulosic biomass particles with an aqueous solution of a water soluble polymer having a flow resistance, followed by agitation, thereby disrupting cellulose polymer chains to separate from one another through efficiently transferring the mechanical force generated by the agitation to cellulose polymer chains (pamphlet of PCT International Publication No. 2009/124072); and the like were proposed.
However, according to the technique (1), an apparatus for providing fine particles of a cellulosic biomass as a suspension is complicating. In addition, since a large amount of energy is consumed during employing this technique; therefore, high productivity cannot be achieved. On the other hand, according to the technique (2), a certain level of improvement of hydrolyzability of the cellulosic biomass is found by using water soluble polymer for imparting a flow resistance to the aqueous solution. However, a further modification has been required for improvement of hydrolyzability for putting into practical applications.